Field of the Invention
The current invention relates to means for inhibiting or ameliorating the progression of myopia, particularly in young people, and includes both methods and apparatus. The methods include procedures for the prescription, selection, fitting and supply of contact and spectacle lenses. The apparatus includes stocks, sets or kits of such lenses and to lenses or lens components per se.
In this specification contact and spectacle lenses capable of—or intended for—both correcting central refractive error and inhibiting the progression (increasing severity) of myopia over time are termed ‘anti-myopia’ lenses.
Background and Discussion Of Prior Art
Myopia (short-sightedness) is a disorder of the eye in which accommodation of the natural lens can bring near objects but not distant objects to focus on the central retina, distant objects being focused in front of (anterior to) the retina. That is, the focusing power of the eye is too strong ‘at distance’ for the accommodative power of the eye. The condition is corrected by the use of lenses with negative central refractive power which enable natural accommodation of the lens to focus both near and distant objects on the fovea in the central portion of the retina. Hyperopia (long-sightedness) is a disorder where distant but not near objects can be clearly focused, the condition being corrected by the use of positive power lenses.
Progressive myopia, which is generally considered to be caused by gradually increasing eye length rather than lens power, can be a serious condition that leads to increasing visual impairment despite the use of successively stronger corrective lenses. Some countries in Asia are reporting that more than 80% of youths aged 17 years suffer from myopia and that many are likely to have or develop the progressive condition.
It is generally agreed that normal eye development—called emmetropization—is regulated by a feedback mechanism that controls eye length to allow good central focus by accommodation at both distance and at near—called emmetropia—during animal growth. It is therefore assumed that, in progressive myopia, this feedback mechanism goes awry and causes the eye to continue to lengthen excessively even though good corrective lenses are used. Many conflicting theories have been advanced about the nature of the feedback mechanism and, thus, many different treatments for progressive myopia have been proposed.
It has been proposed, for example, that the feedback mechanism controlling eye growth is somehow upset by deficiencies in the accommodative effort of the eye due to excessive near work. The deficiency is considered to manifest as lag of accommodation (imprecise and insufficient accommodation) at near resulting in defocus, which stimulates further undesirable axial elongation of the eye. Bifocal lenses and PALs (progressive addition lenses) in spectacles were thus employed to relieve the accommodative stress and defocus in the hope that the stimulus for elongation would be removed. However, data from clinical studies showed poor efficacy over the use standard refractive correction using negative power lenses.
U.S. Pat. No. 6,752,499 to Aller teaches prescribing commercially available concentric bifocal contact lenses for myopic eyes that also exhibit near point esophoria to control the progression of myopia. Both distance-center and near-center contact lenses were employed. These lenses, in which both distance and near zones lie within the normal pupil diameter or ‘optic zone’ of the lens, have the disadvantage that they present two central images to the retina at all times so that image quality is always degraded. In addition, the success of such treatment methods appears to be limited and variable.
In U.S. Pat. No. 6,045,578 to Collins et al. propose that emmetropization is regulated by the degree and direction of spherical aberration present at the fovea. It was proposed that young myopes have higher levels of central negative spherical aberration which promotes inappropriate eye growth and that the use of therapeutic lenses to impart positive central spherical aberration will inhibit excessive axial growth and thus the progression of myopia. We are not aware of the publication of any significant comparative trial using lenses advocated by Collins et al for controlling the progression of myopia. However, we note that the additional spherical aberration further degrades central image quality for both near and distance vision and is, as before, inherently undesirable.
In WO 200604440A2, Phillips et al suggest that simple defocus at the fovea for both distance and near vision inhibits excessive eye growth. They therefore teach the use of a bifocal contact lens that simultaneously provides the central retina with (a) clear vision for both distance and near and (b) myopic defocus for both distance and near. Again, we are not aware of significant published trials reporting the efficacy of this approach and note again that central vision is degraded.
In contrast to the above, U.S. Pat. No. 7,025,460 to Smith et al discloses compelling results of animal trials which demonstrate that it is the nature of the peripheral image, not the central image, that provides the feedback stimulus for emmetropization. (These trials and experiments have been published in prestigious peer-reviewed scientific journals and have received widespread acceptance in the scientific community.) Thus, Smith et al, teach that control of off-axis focus by manipulation of the curvature of field to move the peripheral image progressively in front of the peripheral retina with increasing peripheral angle provides a method of abating, retarding or controlling the progression of myopia. Lenses that manipulate the peripheral image in this way are therefore called ‘anti-myopia’ lenses as they inhibit myopia progression as well as providing correction of central refractive error. Smith et al noted that hypermetropia or hyperopia (impaired near vision caused by insufficient eye length) could be addressed by manipulation of the curvature of field to move the peripheral image progressively behind the peripheral retina.
International patent application WO/2007/146673 by Holden et al disclosed two-zone anti-myopia lenses that are more easily designed and manufactured than those which manipulate peripheral curvature of field in the manner taught by Smith et al. In such lenses, the central zone that provides the refractive correction needed for good central vision approximates the pupil diameter and is surrounded by a single-focus therapeutic peripheral zone having a refractive power tailored to move at least portion of the peripheral image in front of the retina.
While we have confirmed the work of Smith et al and agree with Holden et al that a two-zone anti-myopia lens is easier to design and manufacture, the implementation of the Smith/Holden teachings in practice is still not straight forward as it requires instruments, training and facilities for the measurement of peripheral refraction that are not widely available, especially in the less affluent countries where progressive myopia is a severe problem. The correct prescription of anti-myopia lenses with a peripheral zone tailored to a patient's eye requires, for example, (i) a peripheral refractometer that is capable of reliably determining peripheral focus, (ii) trained professionals who can use such refractometers with appropriate skill and who can accurately specify the characteristics of corrective lens required for a particular patient, as well as (iii) the presence of a lens manufacturing facility that is capable of making custom lenses with prescribed central and peripheral profiles to order. The associated costs may well put such anti-myopia lenses beyond the reach of those most in need, despite being simpler to design and specify than the ‘progressive’ anti-myopia lenses of Smith et al.
At this point, three matters of terminology need to be clarified: how the severity of myopia is indicated, the difference between conventional bifocal lenses and anti-myopia lenses, and, the use of absolute and relative terms to indicate the peripheral power of a lens.
First, it is conventional to refer to a patient as, say, a ‘minus 3 D myope’ meaning that the patient needs or wears −3 Diopter (“D”) corrective lenses. This can be confusing because the patient has a +3 D refractive error and could—with some logic—be called ‘a +3 D myope’. Since the conventional terminology is entrenched, it will be used herein but care will be taken herein to indicate whether the refractive error of the eye or the power of the corrective lens is intended.
Second, a conventional bifocal lens has two central optic zones of different refractive power, one enabling good central distance vision and the other enabling good central near vision. In bifocal spectacle lenses, the near zone is formed in the lower portion of the lens and the distance zone is formed in the central and/or upper portions of the lens. This allows the desired zone and image to be automatically selected by normal eye movement so a single image is presented to the eye. Because conventional bifocal contact lenses are located on the cornea and move with the eye, both the distance and near zones are located in the central portion of the lens that approximates normal pupil diameter. Thus, both the corrected distance and near images are always presented to the fovea simultaneously and it is left to the brain to direct attention to one or the other, but each image is necessarily degraded by the other. Anti-myopia lenses are not inherently—or even preferably—bifocal in that they are not concerned to provide good near and distance central vision using different central optical zones. Instead, anti-myopia lenses normally have a central refractive zone to correct central myopic refractive error and provide good central vision and a peripheral ‘therapeutic’ refractive zone outside the central zone to inhibit continued eye growth. However, anti-myopia lenses can be bifocal, in which case they would have two central zones like a conventional bifocal lens in addition to the therapeutic peripheral zone.
Third, the difference between the refractive power of the central and peripheral zones of an anti-myopia lens is often referred to as ‘peripheral defocus’ because it is conventional to specify lenses in terms of a base corrective refractive power applied to the whole optic zone and to regard a different power in the periphery to be a modification of the base power. Thus, when the peripheral refractive power is less negative than the central power, the corrective lens is said to have peripheral ‘myopic defocus’ and, when the peripheral refractive power is more negative than the central power, the lens is said to have ‘hyperopic defocus’ in the periphery. This is confusing if the change in peripheral power improves focus in the periphery. On the other hand, as the peripheral defocus of many anti-myopia lenses is increased to ensure that the peripheral image is in front of the retina, these lenses may cause focal error or blur in the peripheral retina. In this specification, ‘peripheral defocus’ will be used conventionally for the relative difference between peripheral and central refractive power of an anti-myopia lens and ‘peripheral power’ will indicate the absolute refractive power in the periphery of the optic zone of a lens. It will be appreciated, however, that peripheral defocus and peripheral power are essentially equivalent since one can readily be derived from the other if the central power of the lens is known. It should also be noted that the peripheral defocus may be different for different radial distances on a lens if the peripheral power and/or central power of the lens is not constant with radius. Finally, the peripheral mis-focus perceived by a patient fitted with an anti-myopia contact or spectacle lens will be called ‘blur’ or ‘peripheral blur’.